Matt Zukowski Matt Zukowski - 1 year ago 212
Javascript Question

Why is lodash.each faster than native forEach?

I was trying to find the fastest way of running a for loop with its own scope. The three methods I compared were:

var a = "t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t".split();

// lodash .each -> 1,294,971 ops/sec
lodash.each(a, function(item) { cb(item); });

// native .forEach -> 398,167 ops/sec
a.forEach(function(item) { cb(item); });

// native for -> 1,140,382 ops/sec
var lambda = function(item) { cb(item); };
for (var ix = 0, len = a.length; ix < len; ix++) {

This is on Chrome 29 on OS X. You can run the tests yourself here:

How is lodash's
almost twice as fast as native
? And moreover, how is it faster than the plain
? Sorcery? Black magic?

Answer Source

_.each() is not fully compatible to [].forEach(). See the following example:

var a = ['a0'];
a[3] = 'a3';
_.each(a, console.log); // runs 4 times
a.forEach(console.log); // runs twice -- that's just how [].forEach() is specified

So lodash's implementation is missing an if (... in ...) check, which might explain the performance difference.

As noted in the comments above, the difference to native for is mainly caused by the additional function lookup in your test. Use this version to get more accurate results:

for (var ix = 0, len = a.length; ix < len; ix++) {

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